The present invention relates generally to DC-DC power converters. More particularly, the present invention relates to protection circuitry for single-stage, non-isolated buck-boost power converters with respect to input surge conditions.
Non-isolated power supplies for lighting sources, such as light emitting diode (“LED”) arrays, remain a popular choice for low to medium power levels (e.g., 10 W-30 W), due at least in part to their relative simplicity of design and low cost. Among non-isolated drivers, a buck-boost power factor correction (“PFC”) topology provides a number of well-known advantages including an ability to produce wide output voltage, relatively simple control and moderate power factor operation.
By way of further comparison, a single-stage non-isolated buck boost type LED driver is an attractive option due to its smaller size, lower cost and higher efficiency with respect to a single-stage isolated LED driver and a dual-stage (PFC+DC-DC converter) LED driver. The single-stage non-isolated buck boost type LED driver functions simultaneously as a PFC stage and AC-DC current controller.
A conventional LED driver 10, an example of which is represented in FIG. 1, may typically include one or more input surge protection devices 12 and electromagnetic interference (hereinafter “EMI”) filtering circuitry 13 provided between a sinusoidal AC source 11 and an input rectifier 14. The rectifier and a subsequent power factor correction (hereinafter “PFC”) stage 15 convert an AC input to a high voltage DC output, further regulating the power factor and total harmonic distortion. The PFC stage 15 further regulates the output voltage and current across an output capacitor C2 and thereby to a load 16.
Energy storage elements are conventionally used in DC-DC power converters for filtering, or “smoothing,” pulsating DC input from, e.g., the rectifier stage to the PFC stage by absorbing peak currents and ripple currents while providing a relatively constant DC voltage output. However, such an energy storage element such as capacitor C1 as illustrated in FIG. 1 cannot be very large (usually 100 nF-330 nF) due to inherent limits of the power factor correction application, which means that the capacitor C1 cannot absorb enough energy during certain (e.g., combination wave) high energy surge conditions, even with the assistance of the input surge protection stage 12. As a result, a large voltage spike could appear across the filtering capacitor C1, as well as across switching elements and other sensitive components associated with the PFC circuit. This high voltage spike could easily damage such components and cause total system failure.
Boost-type PFC circuits as are conventionally known in the art may provide better surge immunity than non-isolated buck-boost type PFC circuits. In such topologies, the presence of the surge protection devices 12 at the front end may be sufficient to enable the circuit to survive a combination wave surge (e.g., 6 kv, 1500 A). A primary reason for this is that the boost converter has a relatively large output capacitor C2 that can absorb the excess energy that passes the conventional surge protection stage 12. Since the output capacitor C2 in such topologies typically is large (e.g., 20 uF to 100 uF, or about 200× more than the filtering capacitor C1), the voltage across the output capacitor will not change much due to the energy surge spike. As a result, the voltage across the PCT circuit components will also not be dramatically affected by the surge condition.
Therefore, it would be desirable to provide improved surge immunity for non-isolated buck-boost type PFC circuits. Amore particularly, it would be desirable to provide circuitry to help such PFC converters to survive high energy combination wave surges, by helping to absorb the extra energy that passes after the surge protection stage. As previously mentioned, it is not enough to simply increase the size of the filtering capacitor C1 as that will disable the necessary PFC circuit functionality.